Among various light-based exposure systems used in photolithographic processes, exposure systems employing EUV (extreme ultraviolet) light are the subject of substantial current development activity. In these systems, EUV light is used as the exposure light, as described for example in U.S. Published Patent Application No. 2005/157384 A1.
In configuring an exposure system employing EUV light (such a system is called an “EUV lithography” system, abbreviated “EUVL” system), there are no known materials that both transmit EUV light and exhibit sufficient refraction to such light to be useful as EUV lenses. Consequently, in an EUVL system, the constituent optical systems must be configured using EUV-reflective mirrors. In certain situations, the mirrors can be oblique-incidence mirrors or multilayer-film mirrors. In EUV light reflected from a multilayer-film mirror, the respective phases of multiple fronts of weakly reflected light at layer interfaces are superposed constructively to yield high overall reflectance.